1. Field of the Invention
This invention relates to a method for covering the surface of a molten metal, and in particular relates to a method for covering the surface of a molten metal in order to maintain the temperature thereof and to prevent oxidation of the surface thereof. The invention also relates to the covering material used.
2. Description of the Prior Art
In the casting of zinc, aluminum, or alloys of these metals, etc., ingots of the metal or alloy are heated in a melting furnace to obtain a molten metal which is kept in a tank, variously called a molten metal chamber, a molten metal tank, and so on. From here a desired smaller quantity of the molten metal can be obtained when required by ladling, or tapping off from the bottom of the tank. While the molten metal is stored in the tank, however, it goes without saying that it is essential to prevent heat losses from the surface of the molten metal, and thus to prevent any lowering of the temperatures thereof. Particularly in the case where the molten metal is removed by ladling, it is necessary to construct the molten metal tank with an open upper portion to allow access for the ladle, and so the potential heat losses from the surface of the molten metal are great, and so the temperature must be maintained by some means. Further, quite apart from the problem of maintaining the temperature of the molten metal, there is the problem that with certain metals, such as aluminum, the exposed upper surface of the molten metal reacts with the oxygen in air to produce an oxide of the metal, or with the water vapor in air to produce an oxide of the metal, and hydrogen. And if there is a considerable quantity of the oxide, a reduction in the quantity of the cast product is inevitable, while the hydrogen produced, if it gets absorbed into the molten metal, produces the defect known as gas porosity in the cast product.
As one method of solving these problems associated with the temporary storage of molten metal, a commonly known method has employed covering the exposed upper surface of the molten metal with a flux comprising a halide of an alkaline earth metal, or the like, while Japanese patent publication No. 54-20447 discloses a method of covering the surface of the molten metal with ceramic fibres. The former of the two methods, however, suffers the drawback that the flux is hygroscopic, and so, according to its method of storage and use, it may actually become a source of water for the molten metal, and, instead of preventing the oxidation of the molten metal, it may actually promote oxidation and the absorption of hydrogen gas. Also, in the latter method, the ceramic fibers effectively maintain the heat, but they tend to be ladled up together with the molten metal, and their subsequent removal is troublesome, on top of which, the ceramic fibers are easily mixed into the molten metal, so that, for example, after the molten metal has been used up, the ceramic fibers in the tank have to be carefully completely removed by means such as a vacuum suction pump, before a new load of molten metal can be placed in the tank. Thus the latter method suffers the drawback of requiring considerable care and attention in use.